Any device that requires polarized light, uses one or more polarizers. Polarizers have many industrial applications. For example, polarizers may be utilized in electro-optical modulators and laser subsystems. In essence, a polarizer eliminates an undesirable light component of a first polarization, and allows a desirable light component of a second polarization to pass through.
Of particular interest is the use of polarizers as in-line modules in optical fibers. Previously known in-line polarizers typically comprise an assembly with a first lens following a first optical fiber for collimating the light emerging from the fiber. The collimated light then passes though a polarizer plate and is then focused by a second lens into a second optical fiber. The main disadvantage of this type of polarizer is that it is relatively expensive and difficult to construct. Furthermore, the lens-based polarizer interrupts the optical fiber leading to optical loss, undesirable reflection, and reduced stability. Finally, the lens-based polarizer introduces a device into the fiber that is larger than the fiber, thereby causing potential space issues.
One attempt to solve the above problems was the development of another in-line fiber polarizer that was constructed by wrapping the optical fiber in several loops around a circular member before allowing the fiber to continue on its way. This arrangement eliminated some of the drawbacks of the previously known lens-based polarizer—for example this was a true in-fiber device that did not interrupt the fiber with a larger device. However, the coil-based polarizer suffered from another significant drawback—the coil element around which the fiber needed to be wrapped was typically many centimeters in diameter making the coil-based polarizer very bulky and difficult or impossible to use in many applications.
A novel in-fiber polarizer, that advantageously solved all of the problems of the prior art polarizers was disclosed in a commonly assigned U.S. Pat. No. 6,721,469, issued on Apr. 13, 2004, and entitled “Chiral In-Fiber Adjustable Polarizer Apparatus and Method” (hereinafter the “Adjustable Polarizer patent”), which is hereby incorporated by reference in its entirety. That novel polarizer worked with circularly polarized light and utilized a fiber component that effectively functioned as a quarter-wave plate to convert circular polarization into linear polarization over a relatively narrow frequency band. The fact that polarization conversion only happens across a narrow frequency band, is one of the chief limitations and drawbacks of quarter-wave plates and quarter-wave plate-type devices. In addition, since most practical applications utilize linearly polarized light (for example, light transmitted through standard polarization-maintaining fibers), the polarizer disclosed in the Adjustable Polarizer patent required conversion of linearly polarized light into circularly polarized light prior to entering the polarizer.
Another novel in-fiber polarizer solution was provided in the commonly assigned U.S. Pat. No. 7,095,911 issued on Aug. 22, 2006, and entitled “Chiral In-Fiber Polarizer Apparatus and Method” (hereinafter the “Chiral Polarizer patent”), which was directed to a chiral in-fiber polarizer implemented in a chiral fiber structure having a novel pitch variation along its length between the entry and exit ends in accordance with a predetermined desirable pitch profile that may be advantageously selected to correspond to one or more predetermined pitch configurations. In accordance with the inventive embodiments disclosed in the Chiral Polarizer patent, at least one of various parameters of the chiral structure, including, but not limited to, the core and cladding refractive indices and sizes, and the pitch profile, may be configured and selected to substantially eliminate the undesirable polarization component of the incident light by achieving an optimized extinction ratio within a desired spectral range. The in-fiber chiral polarizer disclosed in the Chiral Polarizer patent, was also configurable into an arrangement enabling significant reduction of insertion loss of the incident light entering the entry end thereof.
However, the various embodiments of both solutions disclosed in the above-discussed Adjustable Polarizer and Chiral Polarizer patents, involved the use of chiral fiber structures which may be difficult to fabricate in-line with conventional fibers (and which would typically be spliced in-line with conventional fibers during use), and which, in certain embodiments thereof, would need to utilize one or more optical fiber couplers (such as disclosed in the commonly assigned U.S. Pat. No. 7,308,173, issued on Dec. 11, 2007, entitled “Optical Fiber Coupler with Low Loss and High Coupling Coefficient and Method of Fabrication Thereof”). Additionally, for certain applications, it is useful to have an in-line polarizer configured to operate at only at least one predetermined wavelength.
It would thus be desirable to provide an in-line polarizer that does not interrupt an optical fiber with a larger structure. It would additional be desirable to provide an in-line polarizer configured to operate at only at least one predetermined wavelength. It would further be desirable to provide an in-line polarizer that may be readily fabricated and/or positioned in-line with a polarization maintaining optical fiber. It would also be desirable to provide an in-line polarizer that is inexpensive and easy to fabricate.